Typically, the use of LEDs as an AC powered light source requires a driver to convert AC line power to a regulated DC power for maintaining constant light output. Such an LED driver is a self-contained power supply that has output matched to the electrical characteristics, such as forward voltage and driving current, of the LED. Usually, the driver is built on an individual PCB and connects to an LED PCB which contains the array of LEDs.
FIG. 1 shows typical circuitry of an AC-DC LED driver using the known flyback converter topology. The circuitry 10 includes an AC power source 12 with live (L) and neutral (N) terminals, and a rectifier circuit 14 that converts the AC, e.g., sinusoidal, input waveform to a DC waveform, e.g., a half wave rectified waveform. The circuit operates such that when the switch Q1 conducts, current is directly drawn from the rectified sinusoid. Energy is stored in the magnetizing inductance of the primary winding of transformer T1. The rectifying Zener diode D1 is reverse biased and the LED current is supplied by the secondary capacitor Cout. When Q1 turns off, diode D1 conducts and the stored energy is delivered to the secondary winding of transformer T1 and to the output. The controller chip U1, which can be, for example an iW3620 Digital PWM Constant Current Controller for AC/DC LED Driver, manufactured by iWatt, Inc., regulates the LED current by comparing the information about the secondary output voltage and LED current, which information is reflected via the auxiliary winding of transformer T1, to a constant reference and, based on the comparison, adjusting the duty cycle of switch Q1.
AC-DC drivers such as the one shown in FIG. 1 are complex and bulky circuits. Proper electromagnetic interference filter design is needed to suppress conducted interference that is generated by the driver or by other equipment. The lifetime of such circuits is usually limited by the electrolytic capacitors used in such circuits.
On the other hand, LED lighting products have recently been developed in which a number of series-connected LEDs are operated with built-in LED driving circuitry that can operate from an AC power source directly. The built-in driving circuitry has an AC input connection that is configured for being received in a standard AC outlet. A disadvantage of such products, however, is that the AC line voltage is sinusoidal, and low-frequency AC LED systems conventionally operate in a rectified mode, that causes the AC-driven LED to be turned off and on at a rate that is double the line frequency. FIG. 2A schematically illustrates such a known AC-driven LED. In this known circuit, a bridge rectifier BR1 converts the AC output from a power source VI to a pulsating DC voltage, which forward biases a string S1 of series-connected LEDs, causing all of the LEDs in the string S1 to conduct, and thus emit light. A resistor R1 limits the current through the LED string S1.
FIG. 2B shows the light output of such an AC-driven LED. It can be seen that the overall off-time is approximately 40%. This percentage of off-time may, under some circumstances, be noticed by an observer as a flicker and can be a drawback to the acceptability of LED lighting by certain consumers.